Atomic structure of bacteriophage Sf6 tail needle knob

Area of Science:

• Virology
• Structural Biology
• Bacteriology

Background:

• Podoviridae are double-stranded DNA bacteriophages with short tails.
• P22-like phages possess a tail needle for host cell DNA ejection.
• The Sf6 phage tail needle has a C-terminal knob, unique among P22-like phages.

Purpose of the Study:

• To determine the crystal structure of the Sf6 tail needle knob.
• To understand the structural basis of Sf6 phage-host interaction.
• To investigate the role of the tail needle knob in bacterial cell envelope penetration.

Main Methods:

• X-ray crystallography at 1.0 Å resolution.
• Structural comparison with related viral proteins.
• In vitro self-trimerization assays.

Main Results:

• The Sf6 tail needle knob forms a trimeric globular domain with a TNF fold.
• The structure is superimposable with adenovirus and PRD1 phage spike proteins.
• Three molecules of l-glutamic acid (l-Glu) bind at each subunit interface, without affecting stability or trimerization.
• The knob structure suggests a mechanism for threading through peptidoglycan pores.

Conclusions:

• The Sf6 tail needle knob's structure is conserved with known receptor-binding domains but likely functions in direct cell penetration.
• l-glutamic acid binding does not appear to be essential for knob structure or function.
• The tail needle is proposed to be ejected through the bacterial cell envelope, with the knob threading through peptidoglycan pores.